Generally, as compared with cross-linked rubber, thermoplastic elastomers are advantageous in terms of production, since the production requires no cross-linking step. Moreover, the thus-produced thermoplastic elastomers are easy to recycle. In addition, in use, thermoplastic elastomers are generally harmless polymers, by virtue of having no cross-linking agent added thereto.
Examples of such thermoplastic elastomers include styrene-based thermoplastic elastomer, olefin-based thermoplastic elastomer, vinyl chloride-based thermoplastic elastomer, polyurethane-based thermoplastic elastomer, and polyamide-based thermoplastic elastomer. As compared with vulcanized rubber, these thermoplastic elastomers generally have high hardness and high compression set.
Compression set of thermoplastic elastomer can be reduced through cross-linking reaction by the mediation of a cross-linking agent added thereto. However, in this case, hardness increases further.
In contrast, low-hardness moldings can be produced from thermoplastic elastomer into which a plasticizer has been incorporated in a large amount. However, such moldings generally have problems, including reduction in tensile strength and bleeding of the plasticizer from the obtained moldings.
Thus, conventionally, many studies have been carried out in order to solve the aforementioned problems and to produce a thermoplastic elastomer having reduced hardness and suppressed compression set.
For example, there has been disclosed a thermoplastic elastomer composition containing a styrene block copolymer, a low-molecular weight polyolefin-based softening agent, a plasticizer, and a polyolefin-based processing aid (see Japanese Patent Application Laid-Open (kokai) No. 61-243852). However, the disclosed thermoplastic elastomer composition has a hardness A by JIS as high as 50 to 75.
Another thermoplastic elastomer composition containing a hydrogenated product of a block copolymer including at least two polystyrene blocks; a hydrogenated product of a low-molecular weight conjugated diene polymer; and a polyolefin resin has been disclosed (see Japanese Patent Application Laid-Open (kokai) No. 8-41283). However, the disclosed composition has a JIS hardness A higher than 70, although the composition exhibits suppressed compression set and excellent tensile strength.
There has been disclosed yet another thermoplastic elastomer composition (see Japanese Patent Application Laid-Open (kokai) No. 2000-44691), which is produced by sequentially kneading a hydrogenated product of a block copolymer including a poly(aromatic vinyl) block and a poly(conjugated diene) block; a non-aromatic oil; and a polyolefin resin by means of a twin-screw extruder. However, the disclosed composition has a poor tensile strength as low as about 4 MPa, although the composition has a low JIS hardness A of 22 and exhibits suppressed compression set.
As described above, when a thermoplastic elastomer having low hardness and suppressed compression set is produced, the hardness can be lowered to only a limited degree. When the hardness is sufficiently lowered through addition of a predetermined amount of oil for preventing bleeding, tensile strength is poor.